Shear Bond Strength of Zirconia Ceramic to Four Different Core Materials, An in vitro Study

Statement of the Problem: Different materials can be used to reconstruct the core foundation in all-ceramic restorations. Bond strength of the core material to zirconia is an important factor in long-term restoration success. Purpose: The aim of this study was to assess shear bond strength (SBS) of zirconia to four different core materials. Materials and Method: In this experimental in vitro study, 40 zirconia ceramic disks (10×3 mm) were prepared and divided to four groups based on core material. Cylinder shaped core specimens (3×4 mm) of non-precious gold alloy (NPG), zirconia ceramic, natural dentin, and composite resin were prepared and bonded perpendicularly to the zirconia disks using Gillmore Needle Apparatus and dual cure resin cement. All samples were thermocycled for 2000 cycles. To evaluate SBS, the specimens were tested by universal testing machine. Data were analyzed using Kruskal-Wallis test followed by Dunn's test with Bonferroni correction. Statistical significance was set at p< 0.05. Results: The highest values for SBS were achieved in composite resin group (11.58±1.74 MPa) followed by NPG (10.32±0.94 MPa), zirconia (7.3±1.11 MPa) and dentin group (6.53±0.56 MPa). SBS in composite resin and NPG core materials were significantly higher than other core materials (p< 0.05). Conclusion: Composite resin and NPG cores showed significant higher bond strength to zirconia in comparison to dentine and zirconia core materials.


Introduction
There are several methods mentioned in the literature for restoring damaged anterior teeth [1]. Increased demand for esthetic restorations and unpredictable biocompatibility of some metal alloys has attracted attention toward metal-free restorations in recent decades [2] and has turned them to a routine choice in prosthetic treatments [3].
Several materials can be used to compensate the missing tooth structure as a core foundation for extracoronal restorations. The choice of proper material depends on the amount of remaining tooth structure, esthetics, finances, and treatment duration [4]. The non-precious gold alloy (NPG), containing more than 80% copper, first was introduced in 1987. It has optimal physical properties as dental post-cores with simpler preparation and handling compared to nickel-chromium ones [5].
In translucent ceramic crown systems, there is concern about the impact of abutment shade beyond the restoration [6]. For high translucent restorations with a thickness less than 1.6mm, the shade of underlying abutment may affect the final esthetic result [6]. The development of tooth colored post-core systems, such as composite resin or ceramic dowel and core restorations, has improved esthetics [7]. Non-metallic post-cores not only provide more esthetic over metallic posts, but also reduce the risk of corrosion and toxicity. Nowadays, many composite resin systems are available specifically designed for core build-up with more fillers, higher strength, and easier manipulation [8][9][10][11].
The bond strength of core foundation and crown plays essential role in success rate of full ceramic restorations [12]. Previous studies about bond strength of zirconia crowns have been concentrated on surface treatments before cementation and use of various adhesive resins [13][14][15][16]. The use of airborne particle abrasion technique results in improvement of the bond between resin cement and yttria-stabilized tetragonal zirconia polycrystalline ceramics by producing roughness at the zirconia surface. This surface roughness increases the surface wettability and micromechanical retention with luting agents [17].
Several methods including use of phosphoric acid ester monomers like 10-methacryloyloxy-decyldihydrogen-phosphate, zirconia coupling agent, and organic silane have been suggested to improve zirconia crowns bond strengths [18][19]. The phosphate containing monomers behave similar to the silane coupling agents because they allow the copolymerization between the methacrylate group and the monomers of a composite resin system. In addition, they bond with the metal oxides in the substrate with phosphoric acid groups. Carboxylic acid is another monomer, which plays an important role in the bond formation [17]. High amounts of bond strength were reported in the literature when using methacryloyloxy-decyl-dihydrogen-phosph-atecontaining resin cement (Panavia F 2.0; Kuraray) [20].
Bond strength of zirconia crowns to different core materials has been investigated in limited studies [4].
So, the aim of this study was to evaluate shear bond strength (SBS) of zirconia to four different core materials. The null hypothesis of this study was that the core material has no effect on the SBS of zirconia crowns.   The maximal force (MPa) for debonding was recorded. The data was subjected to Kruskal-Wallis test followed by Dunn's test with Bonferroni correction. The significance level was 0.05%.

Results
Mean ± standard deviation values of SBS for all groups are presented in Table 2.
To investigate the normality of the quantitative variable distribution of data, Kolmogorov Smirnov test was used. With no confirmation of normality assumption,   Figure 2 shows that SBS in composite and NPG is higher than zirconia and dentin.

Discussion
Selection of an appropriate material in restoring the core of a damaged tooth depends on the remaining dental structure and esthetic factors [4]. Proper bonding between the core material and the crown guarantees the long term success of a restoration [4]. Recently, zirconia crowns have gained lots of popularity among dentists due to the high strength and esthetic [21][22].
Bonding of zirconia to the core materials is challenging. Chemical structure contains polycrystalline and lack of amorphous glass phase is believed to be the main cause of weak bond of yttria-stabilized tetragonal zirconia polycrystalline ceramics [17]. Several methods have been suggested to improve bonding properties of zirconia, including conventional surface treatments like grinding with diamond burs, air abrasion with aluminum oxide (Al2O3), tribochemical silica-coating, acid etching with hydrofluoric acid, coupling with silane agent, plasma spraying with hexamethyldisiloxane, and combinations of any of these methods [23][24][25][26][27][28][29][30][31][32]. Jiao et al. [33] mentioned that sandblasting of zirconia increased SBS between zirconia and resin cement. So, in this study all specimens were sandblasted with 50µ aluminum oxide under pressure of 3 bars for 15 seconds as noted in similar study [4]. Our results showed significantly higher SBS of zirconia to composite core material and NPG, witch rejecting our null hypothesis. In order to justify these results, we can refer to Al-Harbi et al. [17], which evaluated SBS of yttria-stabilized tetragonal zirconia polycrystalline ceramics to different core materials with the use of three primer/resin cement systems. They pointed out that formation of more surface oxides in metal and composite resin cores might be a cause for stronger SBS in these materials compared with those formed on the surface of the zirconia cores [17].
The high organic content and moisture of natural dentin could be the main reasons of weak bond between zirconia and dentin core, these findings are in agreement with other studies [34][35]. Although dentin bond strength is lower than other core materials, it does not mean the bond is insufficient [4]. In cases which the dental structure is sufficient for placement of a zirconia crown, the tooth structure should not be removed to create stronger bond to the zirconia crown.
Frattes et al. [4] reported that metallic alloys and zirconia core materials had significantly higher SBS compared to dentin. The SBS value of zirconia core in Frattes et al. [4] study was higher than ours. This differ- ence may be attributed to the different methods of air abrasion before cementation. Variables like particle size and shape, incidence angle, moisture and pressure of air abrasion system might play an important role in SBS values [27]. Although zirconia showed higher SBS values than dentin in our research, this difference was not proved to be statistically important.
Similar to our findings, Al-Harbi et al. [17] showed the mean zirconia SBS to metal (nickel-chromium) and composite core material were higher than zirconia core and there was no significant difference between nickelchromium and composite. This study showed higher SBS of composite resin cores compare to zirconia cores, and there was no significant difference between composite and NPG SBS. Copolymerization between the cement monomers and the composite resin at the interface probably results in greater bond strength of the composite resin core material [4]. Due to the same SBS of composite resin and metal cores when bonded to zirconia crown, using composite core in esthetic zone seems reasonable.
Prior to bonding, Clearfil ceramic primer was used to prepare zirconia disk surfaces according to the manufacturer's instructions. This primer contains 3-methacryloxy propyl trimethoxy silane mixed with 10-methacryloyloxy-decyl-dihydrogen-phosphate monomer. By using this primer, chemical bond is formed between zirconia and aluminum oxide substrate and the phosphate ester group of this monomer [36][37][38]. These findings are in agreement with other studies [17].
This study was an in vitro study. Confounding factors like masticatory forces, salivary flow, and fatigue should also be considered to stimulate oral cavity condition and obtain more valid and reliable results for clinical practice. There are few studies about the effect of different types of core material on the SBS values of zirconia crowns. Therefore, more studies with different core materials, bonding systems, aging process and debonding forces is recommended.

Conclusion
Regarding the limitation of this study, the effect of different core materials on the SBS of zirconia was significant. Considering the importance of bonding between crown and core, composite and NPG could be recommended as foundation core material.